When the load exceeds 120 to 130 g/cra2 the friction coefficient at rest becomes almost equal 

 to the friction coefficient in motion and with further addition of weight, hardly differs. An interest- 

 ing observation was made by Alekseev in taking off an airplane weighing 5. 5 tons from a snow air- 

 drome at an air temperature of -25°. With the ski area being 3 . 10 x 0.70 (2. 2 m'^), the effort, 

 according to the dynometer proved to be 2. 2 tons, which corresponds to a 0. 40 coefficient of 

 friction. Thus the laboratory experiments of Arnold-Aliabev are confirmed. 



The fact that the external friction of moist ice is considerably less (1/2 or less) that the 

 external friction of dry ice, explains the following phenomenon. A ship grounded on the ice and 

 imable to get off shortly after becoming grounded, in spite of a number of adopted measures — 

 reversed engines, rocking the ship, throwing anchors out on the ice, etc. , frequently becomes free 

 of the ice after several hours due to the action of reversed engines exclusively. The hull of the 

 ship, especially of a metallic one, is always warmer than ice. Aside from this, the ice under the 

 ship undergoes pressure. The combined action of temperature and pressure melts the ice at the 

 points of contact and the dry friction becomes moist. As a result, less force is necessary to free 

 the ship from the ice. Understandably, what has been said relates to the summer. 



LITERATURE: 67, 77, 145. 



Section 81. Fatigue 



It is known that metallic structures loaded with fluctuating loads, under certain conditions 

 lose their ability to withstand the deformation and are destroyed due to "fatigue. " Fatigue frac- 

 tures differ from the usual fractures due to deformations — breakage — and resemble the fractures 

 of frangible substances. The deformations caused in ice by the loads transported over it also 

 creates changes in the internal structure of the ice. These deformations consist of the extrusion 

 of brine and air bubbles from the ice which makes the ice more monolithic, and of the formation 

 of a multitude of tiny though noticeable cracks in the ice which weaken it. As compared with 

 metals, ice at usual temperatures possesses exceptional properties: plasticity, fluidity, and the 

 ability to regelate. In connection with this, after the pressure ceases, the weakening of the ice is 

 compensated for by a more rapid formation of ice near the more dangerous cracks, i. e. , cracks 

 completely through and nearly through and the ice gradually re-establishes its mechanical prop- 

 erties, which is impossible for metals. Thus, the term "aging" is more applicable to metals 

 whereas the term "fatigue" is more applicable to ice. 



Hence, the movement of heavy machine loads over ice should be curtailed regularly for a 

 certain period of time when heavy use is being made of the ice crossing use. 



LITERATURE: 67. 



202 



